Light Sensor switch

I'm trying to build a circuit, using a LDR or PD, PT etc

I want the sensor to detect a hand waved across the front of it, but not false trigger.

So it would need to detect a drop in light, and only activate the output a second later if the light is back to normal.

How could i design this circuit?

Thankyou.

This could probably get a little complicated, due to your requirements. Have you considered using a low power laser beam as your threshold? Or possibly infared?

If you are using ambient light and not defining what "normal" is, then I suggest you charge a capacitor with a voltage proportional to the ambient light so when the light decreases, you can compare the decrease with the stored voltage. You will need hysterysis in the comparator in order not to have false triggering.